The present invention is directed to the field of image processing, and more specifically to graphics data processing and output data calibration in a desktop computer-to-imagesetter/flexo-printer system.
Data calibration is of interest because of press gain variances with various image presentations. Prior to digital imaging, press gain was compensated by a press operator who adjusted the press settings throughout a series of proofing runs, until a desirable print was obtained. With the introduction of computers into printing, and with computer assisted imaging, calibration for a specific "run", i.e., image to be printed, has been attempted by electronic and computer calibration of the image data itself. This calibration changes the true image data to translated data, which when used by the printing equipment produces a true reproduction of the original image.
Film and plate printers, including flexo-printers, typically received graphics information (image data) from an imagesetter. An imagesetter is a processor based machine which images films or plates to be printed on a printer, such as a flexographic press. The imagesetter images on a bit by bit basis. Its ability to image is independent of image data word length, as it serially processes. Image data provided to the imagesetter often comes from a desktop publishing computer.
Desktop publishing and binary printing processes operate with digital information and digital graphic presentations from the press. Continuous gray level information is simulated using halftone dots. Halftoning is a computer graphics technique for displaying an image, with gray levels, on a binary imaging device (such as a flexographic printer, i.e., flexo-press) in which the gray levels are approximated by variable-sized black and white dots. The image presentation is achieved by changing percentage area coverage from region to region.
A gray code, in computer mathematics, is a binary code in which sequential numbers are represented by binary expressions, each sequential one of which differs from the preceding by one place, only. Gray level is the value associated with a pixel in a digital image, representing the brightness of the original scene in the vicinity of the point represented by the pixel. This translates to a direct relationship to dot size or percent area covered. Gray scale is an optical pattern in discrete steps between light and dark bearing on resolution.
Color images are created by combining gray scale values of a number of selected component colors. Typical component colors, often selected in the industry for hard copy print operations, may be cyan, magenta, yellow and black (CMYK). An individual component color, such as cyan is represented with digital information being a series of gray values represented by 8-bit numbers ranging from 0 to 255. A CMYK image can be generated through the combination of four gray scales images, one for each component color (CMYK).
The Flexographic Trade Association (FTA) in conjunction with the International Standards Organization (ISO) have standardized a gray scale of 0 to 100 to indicate tone or percentage of area covered from 0 to 100 percent. Consequently, 8-bit "words" which have a numbering capacity of 0-255 are quite adequate for defining gray scale values. As a result many, if not most, of the commercially available graphics programs provide 8-bit image data. While some of these commercial graphics programs have incorporated output scaling to compensate for press gain, such scaling calibration is done on 8-bit graphics data which results in lesser quality imaging at the press because of gray scale tonal loss.
As an alternative to these methods, attempts have been made to calibrate the imagesetter itself to compensate for press gain. An imagesetter is considered to calibrated if each continuous tone gray level in the computer produces a halftone dot of the same gray value at the press. This calibration adjustment has been attempted with the use of a "cutback" curve. This cutback curve is an analog plot (curve) of instantaneous gray level reduction as applied to the 8-bit image data being processed by the imagesetter, i.e., the cutback curve results in each dot gray value being downwardly adjusted as function of the nearest 8-bit level approximation to the cutback curve.
While this has produced enhanced results at the press over no calibration, there is again an inherent gray level tonal loss which reduces press image quality. Over any range in the cutback curve, where the slope is less than 1 (45 degrees), the application of a cutback curve will result in 2 different input gray levels being assigned to a same 1 output gray level.
Gray level loss is particularly detrimental when a cutback curve is employed to alter 8-bit digital data for flexographic printing. With such a curve, in the gray scale range of 0 to 8 (percent) the average slope is about 1/4. The result is that each "4 input" gray levels are reduced to "1 output" gray level. Hence, the twenty 8-bit gray values that exist between 0 and 8 percent are reduced to 5 output gray values.
A standard accepted by the ISO as the minimum (color difference detectable to the eye is known as "delta" E. One delta E is a "noticeable visual color difference". Therefore, to render an image on a press with a continuous tone appearance, it is required that 2 digital gray values produce less than 1 delta E difference on the printed sheet. With flexographic printing, the 5 gray values between 0 and 2 percent on the gray scale correspond to a delta E range of about 10. This means that each 8-bit digital gray level difference corresponds to a 2 delta E difference on the printed sheet, and produces a quite distinct step pattern.
What is desired is a method and apparatus for calibrating an imagesetter output to compensate for press gain in printing digital images.
What is secondly desired is such a method and apparatus which operates apart from the imagesetter internal data processor.
What is even further desired is a method and apparatus for effecting such imagesetter calibration by the manipulation of the digital image data prior to being input to the imagesetter.
What is also desired is such manipulation of the digital image data which does not cause a degradation at press of the properly calibrated printed digital image over the entire gray scale.